This invention pertains to a method and apparatus to characterize and classify neutral particles based on the non-translational motion of said particles in a directionally oriented electric field which as a function of time changes its orientation in space. More particularly, a method and apparatus are disclosed whereby the direction and magnitude of the non-translational motion relative to the directionally varying external field may be determined.
U.S. Pat. No. 4,326,934 discloses that non-uniform electric fields induce translational and rotational motions in neutral particles and is incorporated herein by reference. The translational and rotational motions of yeast in an alternating field was observed in 1971. Pohl, H. A. and Crane, J. S., Biophys. J. 11,711 (1971). The translational motion toward and subsequent stacking of cells at the two electrodes producing the AC field was the predominant phenomenon observed. While so stacked, it was noted that occasionally individual cells would rotate about an axis perpendicular to the applied field lines. The cells rotated at a rate of several revolutions per second in response to an AC field frequency in the range of 10.sup.2 -10.sup.6 Hz. This rotational phenomenon was found to be dependent upon the frequency of the external AC field. As the frequency of the applied field was varied individual cells within the stacks of cells at the electrodes were observed to start and stop their rotational motions over a relatively narrow frequency range. These observations gave rise to the term cellular spin resonance (CSR) to describe this phenomenon.
In addition to the cellular rotation observed in stacks of cells, lone single cells were occasionally observed to demonstrate the same phenomenon. The frequency of such a response for live single cells was, however, distinctly different from that observed for cells stacked at the electrodes. Observation of single cells was, however, hampered by the translational motion of such cells toward the electrodes due to dielectrophoretic forces.
The observed phenomenon has been found to be dependent upon the conductivity of the medium, the intensity of the applied field, with the cell type, with the phase of the cell life cycle, and the presence of trace chemicals which affect cells. In addition, it has been postulated that the rotation of live single cells is, at times, due to the interaction of a natural cellular dipole oscillation and the applied field. Natural Oscillating Fields of Cells by Herbert A. Pohl in Coherent Excitations in Biological Systems Ed by H. Frohlich and H. Kremer; Springer Verlag, N.Y. (1983) pgs. 227-238. Other phenomena associated with cells which as yet are unknown may also contribute to non-translational motion. Exposure to complex electric fields may be important in the further characterization of cellular phenomenon.
The characterization of the non-translational motion of neutral particles has heretofore been limited to the measurement of the frequency at which rotation is observed and by a general description that such rotation occurs about an axis perpendicular to the applied electric field. In addition, detailed and continuous observation of single neutral particles has been hampered by dielectrophoretic translational motion. The need therefore existed for an apparatus and method which would allow a more refined study of the observed phenomenon both in arrays of neutral particles and for single neutral particles.
The application of rotating electric fields through the use of multi-electrode systems provides useful information about neutral particles which in the past has been difficult or impossible to obtain. For example, it has been known in the art that living and dead cells as well as certain inanimate particles can be made to spin in an alternating electric field generated by two parallel wires. The use of rotating electric fields as described in the present invention has demonstrated that in a certain frequency range of the applied field under identical environmental conditions live cells spin in a contra-field direction while dead ones spin with the rotating field. Moreover, the direction and magnitude of the non-translational response may be correlated theoretically to the sign and magnitude of the effective dielectric constant of the particle. Since the direction and rate of spin of a neutral particle may be determined over a wide range, dielectric properties of animate and inanimate neutral particles with regular or irregular shape can be determined by use of the present invention.